The present disclosure relates to a method for controlling at least one machining device which is coupled to a machine tool by means of an encoder signal, and to a machine tool which has been set up in a corresponding manner.
Although the features of the disclosure are described substantially using digital printing units, it relates to all types of machine tools in which additional units are coupled by means of encoder information (for example by means of an encoder emulation interface of an automation system). Winder subassemblies in printing machines or packaging machines, cross-cutter subassemblies in production machines (paper processing, sheet metal working), stapler subassemblies in printing machines or bookbinding machines and transverse sealing subassemblies in packaging machines are mentioned merely by way of example.
In printing technology, use is mainly made of so-called analog printing units in which the print image is present as a complete template, for example in the form of a printing plate or a printing block. In addition, however, increasing use has recently been made of so-called digital printing units in which the print image is produced in a computation unit and is then applied to the material or to an image transfer cylinder, for example in an inkjet printing method or in an electrophotographic printing method. In digital printing methods, it is difficult, in particular, to synchronize the start and the internal raster, for example print lines, of the print image with the web transport in order to ensure maintenance of register.
Real incremental encoders or incremental encoder simulations are used for this purpose in the prior art. Digital printing units which are coupled to the web transport via incremental encoders are described, for example, in DE 10 2006 009 773 A1 or EP 1 157 837 A2.
With this type of coupling, the received encoder signal must usually be processed (filtered, converted, etc.) so that the encoder resolution matches the printing unit resolution. If a printing unit is operated at 600 dpi (dots per inch), for example, it is expedient if 600 encoder pulses are likewise available per inch of forward feed of the product web or material sheet. However, very complicated post-processing (filtering, interpolation, multiple evaluation, multiplication, etc.) of the received encoder signal is often required for this purpose.
In most cases, dead times or delay times are additionally present in the control path of the encoder signals as far as the machining unit, which times must be compensated for during encoder preprocessing in the additional unit. These times are, in particular, dead times during encoder emulation, dead times when transmitting the encoder signal, delay times caused by filtering of the (noisy) encoder signal and dead times in the additional unit from encoder evaluation to machining.
The dead times or delay times described should be taken into account since otherwise angle errors (for example register errors, cutting errors, etc.) which are dependent on the machine speed result.
In order to reduce speed-dependent errors, a speed can be formed from the (filtered) encoder signal by means of single differentiation and can be used to extrapolate the encoder position. In order to additionally reduce acceleration-dependent errors, an acceleration can be formed from the (filtered) encoder signal by means of double differentiation and can be additionally used to extrapolate the encoder position.
However, differentiation increases the noise component, and results in severe impairment of the extrapolated encoder signal, in particular in the case of double differentiation. Filtering results in a delay which cannot be completely compensated for by extrapolation either: in the case of only speed-dependent extrapolation, a dynamic angle error will form during the complete acceleration phase in acceleration processes. In the case of additional, acceleration-dependent extrapolation, a dynamic angle error will arise during the transition from a constant speed to the acceleration process.
It is therefore desirable to increase the accuracy with which a machining device is connected to the transport of the product web or material sheet by means of an encoder signal.